Many different reasons exist why some people have hearing impairment. As a general proposition, sound entering the outer ear canal does not get transmitted to the inner ear and/or transmitted to the auditory nerve. In many cases a middle ear device that creates vibrations is used to improve the hearing of such persons. One class of these middle ear devices are known as magnetic middle ear implant devices.
The solution to hearing problems caused by middle ear deficiencies may involve implanting a magnet in the middle ear or placing it on the eardrum and causing the magnet to vibrate in response to environmental sounds. The magnet is connected, for example, such that it provides mechanical vibrations to the oval window, either through an adequately functioning portion of the middle ear's ossicular chain to which the magnet is attached, or through an implanted prosthesis carrying the magnet and communicating with the oval window, round window, or other vibration conducting surface.
The magnet is usually caused to vibrate by placing it near to a coil of wire which is energized by the flow of electricity. Once such a coil of wire is energized by the flow of electricity, it becomes an “electromagnet” whose magnetic strength and polarity are based on the direction and strength of the electric current energizing it. If a permanent magnet is placed near this electromagnetic coil, the magnet will be attracted to, or repelled from, the coil.
However, if the implanted magnet is not optimally aligned with the external coil from which the electromagnetic signal propagates, the implanted magnet might not respond adequately. This is very important, as better patient outcomes will result with optimal magnet and coil alignment.
Even with advanced imaging technology, the final coil alignment may not end up in the optimal position. This results in reduced energy transmission to the magnet. In these cases, a new external sound processor may be made with the coil in a different position to try to achieve optimal alignment. In some cases, it may be necessary to make several external processors with different coil positions before the best coil location is finally achieved. This process of making multiple processor assemblies to arrive at the optimal alignment is expensive and requires many visits between the patient and clinician.
What is needed is a means for adjusting the position of the coil in the same device. One way to do this would be to be able to move the coil to a different position. However, the thermoset polymers which are used in ear mold shells are rigid and do not soften when heated. Consequently, they do not deform even when heat is applied and will ultimately crack if attempts are made to move the coil.
One consideration would be to make the shell out of a thermosoftening plastic, also known as a thermoplastic. Thermoplastics are polymers that become pliable or moldable above a specific temperature, and return to a solid state upon cooling. Most thermoplastics have a high molecular weight, whose chains associate through intermolecular forces; this property allows thermoplastics to be remolded because the intermolecular interactions spontaneously reform upon cooling. In this way, thermoplastics differ from thermosetting polymers, which form irreversible chemical bonds during the curing process; thermoset bonds break down upon melting and do not reform upon cooling. Examples of well-known thermoplastics are nylon (polyamide), polyethylene, polypropylene, acrylics, polystyrene, polyvinyl chloride, and Teflon. Thermoplastics are commonly used in well-known processes such as injection molding, blow molding, rotational molding, extrusion and thermoforming.
An advantage of using a thermoplastic for the shell would allow the shell material to soften when heated, which would allow for moving the coil to a new position. When the thermoplastic cools, it would once again regain a rigid state. However, the use of a thermoplastic shell is problematic for two reasons. First, even when heated, the material is very viscous and will not flow into a reverse mold without a significant amount of pressure. This would necessitate the use of much more expensive manufacturing processes to make the shell. The second problem with thermoplastics is that if the shell is heated to allow movement of the coil, then the shell itself would also be heated which would soften it and could result in deforming its shape such that it would not fit properly in the ear canal.
Because of the critical nature of the alignment, there continues to be a need in the art for a better method of aligning the magnet and coil.